The present invention relates to polymer electrolyte fuel cells which comprise an anode, a cathode and a polymer electrolyte membrane provided between the anode and cathode and which is adapted to generate electric power by supplying a fuel gas containing hydrogen to the anode and an oxidizer gas to the cathode.
Attention has been directed in recent years to fuel cells having a high energy conversion efficiency and producing no harmful substance by an electricity generating reaction. Polymer electrolyte fuel cells which operate at a low temperature of not higher than 100xc2x0 C. are known as such fuel cells.
FIG. 4 shows the electricity generating principle of polymer electrolyte fuel cells. A fuel cell 50 is fabricated by arranging an anode 55 and a cathode 56 at opposite sides of a polymer electrolyte membrane 54 of ionically conductive solid high polymer, and further arranging a fuel chamber 57 and an oxidizer chamber 58 at opposite sides of the arrangement. The anode 55 and the cathode 56 are interconnected by an external circuit 59.
The hydrogen H2 contained in the fuel gas supplied to the fuel chamber 57 is separated into hydrogen ions H+ and electrons exe2x88x92 at the anode 55. The hydrogen ions H+ migrate through the polymer electrolyte membrane 54 toward the cathode 56, while the electrons exe2x88x92 flow through the external circuit 59 toward the cathode 56.
At the cathode 56, the oxygen O2 contained in the oxidizer gas supplied to the oxidizer chamber 58 reacts with the hydrogen ions H+ and the electrons exe2x88x92, producing water H2O.
In this way, the cell in its entirety produces water from hydrogen and oxygen and generates an electromotive force.
Since the single fuel cell 50 is small in electromotive force, a plurality of fuel cells 50 are usually connected to one another in series to provide a polymer electrolyte fuel cell device.
For example, FIG. 3 shows a fuel cell device 5 of the polymer electrolyte type which comprises a plurality of fuel cells 50 each in the form of a flat plate and connected in series as fitted to one another into an assembly. The fuel cells 50 connected in series are supplied with hydrogen gas or like fuel gas and air or like oxidizer gas for the fuel cells 50 to generate electric power and deliver the power to the outside.
Each fuel cell 50 of the device 5 is formed with a plurality of fuel gas channels (not shown) extending vertically and a plurality of oxidizer gas channels 53 extending horizontally.
The fuel cell 50 disposed at one end of the device has a fuel gas inlet 51a, while the fuel cell 50 disposed at the other end thereof has a fuel gas outlet 52a. The fuel cells 50 other than these end cells 50 are each formed with a fuel gas supply through bore 51 and a fuel gas discharge through bore 52.
By fitting the fuel cells 50 to one another, the fuel gas inlet 51a and the fuel gas supply through bores 51 are held in communication with one another to form a fuel gas supply passageway, and the fuel gas discharge through bores 52 and the fuel gas outlet 52a are held in communication with one another to form a fuel gas discharge passageway.
The fuel cell device 5 of the polymer electrolyte type is further provided with an oxidizer gas supply manifold 6 on the side thereof where the oxidizer gas channels 53 are exposed for supplying the oxidizer gas to the channels 53.
The manifold 6 has, for example, an opening facing downward and also an opening facing this side, such that the oxidizer gas taken in through the downward opening is sent into the oxidizer gas channels 53.
With the fuel cell device 5 described, the fuel gas is fed to the fuel gas inlet 51a as indicated by a solid-line arrow in the drawing, distributed to the fuel gas channels formed in each fuel cell 50 via the fuel gas supply passageway and subjected to an electricity generating reaction while flowing down these channels. The portion of the fuel gas remaining unreacted and reaching the fuel gas discharge through bores 52 after flowing through the fuel gas channels flows through the fuel gas discharge passageway provided by the bores 52 and is discharged to the outside from the fuel gas outlet 52a as indicated in a solid-line arrow in the drawing.
On the other hand, the oxidizing gas is taken in from the downward opening of the manifold 6 as indicated by broken-line arrows in the drawing, sent into the oxidizer gas channels 53 through the side opening and subjected to the electricity generating reaction while flowing through the channels 53. The portion of the oxidizing gas remaining unreacted and reaching the outlets of the channels 53 after flowing through the channels 53 is discharged to the outside from the outlets as indicated by broken-line arrows.
However, the fuel cell device 5 has the problem that during the generating operation, the water content of the polymer electrolyte membranes 54 decreases to result in reduced ionic conductivity, rendering the cells no longer serviceable as such.
Accordingly we have filed a patent application on a polymer electrolyte fuel cell wherein unreacted fuel gas and unreacted oxidizer gas are subjected to a combustion reaction, and the resulting water is supplied to a polymer electrolyte membrane to wet the membrane (JP-A No. 40179/1999).
This fuel cell nevertheless has the problem that the oxidizer gas supplied to the oxidizer chamber contains organic impurities such as kerosene and methanol, permitting the impurities to reach the surface of the cathode and inhibit the electrode catalytic reaction, lowering the cell voltage.
Accordingly, a fuel cell is proposed which has an air electrode to be supplied with clean air obtained by burning air by a combustion catalytic device for removing impurities from the air (JP-A No. 94200/1995).
However, the proposed fuel cell has the problem of being low in overall efficiency since the fuel gas remaining unreacted for power generation is discarded to the outside without being reused.
An object of the present invention is to provide a polymer electrolyte fuel cell device which comprises a polymer electrolyte membrane prevented from drying and which affords a high cell voltage without impairment and achieves a higher overall efficiency than in the prior art.
The present invention provides a polymer electrolyte fuel cell device which comprises at least one fuel cell 10 having an anode 15, a cathode 16 and a polymer electrolyte membrane 14 provided between the anode and the cathode for causing the fuel cell 10 to generate electric power by supplying a fuel gas containing hydrogen to the anode 15 and supplying an oxidizer gas to the cathode 16. The fuel cell device is characterized by introducing, into a combustion unit 3, the unreacted portion of the fuel gas discharged from the fuel cell 10 and the whole amount of the oxidizer gas to be fed to the cathode 16 to burn the unreacted fuel gas and burn the impurities contained in the oxidizer gas by partly consuming the oxygen contained in the oxidizer gas, and supplying the oxidizer gas discharged from the combustion unit 3 to the cathode 16.
With the fuel cell device of the invention, the unreacted portion of a fuel gas discharged from the fuel cell 10 is supplied to the combustion unit 3, and an oxidizer gas is supplied from outside to the unit 3.
In the combustion unit 3, the hydrogen contained in the unreacted fuel gas and the oxygen contained in the oxidizer gas undergo a combustion reaction to produce water. The impurities contained in the oxidizer gas also undergo a combustion reaction, whereby the impurities are decomposed into water and carbon dioxide. The oxidizer gas to be supplied to the combustion unit 3 contains oxygen in an amount required for the combustion reactions and an electricity generating reaction.
Accordingly, the combustion unit 3 discharges an oxidizer gas which contains water, oxygen and carbon dioxide gas. The oxidizer gas is supplied to the cathode 16, permitting the water contained in the gas to penetrate into the polymer electrolyte membrane 14 and providing the oxygen for the electricity generating reaction. The carbon dioxide supplied to the cathode 16 is discharged to the outside, along with the portion of the oxidizer gas remaining unreacted, without contributing to the generation of power and without producing any adverse effect on the operation of the fuel cell.
In the fuel cell device embodying the invention, the oxidizer containing water is fed to the cathode 16, permitting the water to penetrate into the polymer electrolyte membrane 14 as stated above and thereby preventing the membrane from drying.
Since the oxidizer gas as made free from the impurities by decomposition is supplied to the cathode 16, the impurities present in the oxidizer gas are unlikely to lower the cell voltage unlike the conventional fuel cells.
Furthermore, the unreacted fuel gas is supplied to the combustion unit 3 for reuse, whereby a higher overall efficiency is achieved than in the prior art wherein the unreacted fuel gas is discarded.
A fuel cell has been proposed in which outside air and unreacted hydrogen are supplied to a catalytic combustion device, and the combustion air available from the combustion device is supplied as admixed with outside air to an air electrode (JP-A No. 73911/1997). Since the mixture of the combustion air and outside air is thus supplied to the air electrode, the proposed fuel cell has the likelihood that the impurities in the outside air will lower the cell voltage.
With the fuel cell device of the invention, on the other hand, the oxidizer gas to be supplied to the cathode is free from impurities. This eliminates the likelihood of the impurities lowering the cell voltage.
Stated specifically, the combined volume of the oxidizer gas and the unreacted fuel gas to be supplied to the combustion unit 3 has a hydrogen gas concentration by volume of at least 4.0 vol. % to not higher than 10 vol. %.
If the hydrogen gas is supplied to the combustion unit 3 in an amount smaller than is necessary, the combustion reaction between the hydrogen in the unreacted fuel gas and the oxygen in the oxidizer gas, and the combustion reaction between the impurities in the oxidizer gas and the oxygen therein will proceed insufficiently, failing to fully moisten the oxidizer gas to be fed to the cathode 16 and to effectively decompose the impurities. If an excess of hydrogen gas is supplied, on the other hand, the oxygen in the oxidizer gas to be supplied to the combustion unit 3 is almost entirely consumed by the combustion reactions, reducing the amount of oxygen to be fed to the cathode 16.
Accordingly, it is desired that the concentration of hydrogen by volume be in the foregoing range.
Further stated specifically, the combustion unit 3 is adapted to activate the combustion reactions of the unreacted fuel gas and the impurities with a catalyst.
When thus adapted specifically, the unreacted fuel gas and the oxidizer gas come into contact with a combustion catalyst 32, which activates the combustion reaction between the hydrogen in the unreacted fuel gas and the oxygen in the oxidizer gas, and the combustion reaction between the impurities in the oxidizer gas and the oxygen therein, consequently fully moistening the oxidizer gas to be fed to the cathode 16 and decomposing the impurities more effectively.
With the fuel cell device of the polymer electrolyte type according to the present invention, the oxidizer gas containing water and made free from impurities by decomposition is supplied to the cathode to prevent the polymer electrolyte membrane from drying while enabling the device to give the desired cell voltage without impairment. With the unreacted fuel gas reused, the device attains a higher overall efficiency than those of the prior art.